The present invention relates to processes for the preparation of finely-divided alumina or non-stoichiometric spinel powders based on alumina by the thermal decomposition of ammonium alum or of mixtures of ammonium alum and the hydrated double sulfate of magnesium and ammonium, and more particularly, the present invention permits the production of a product characterized by a particularly fine and homogeneous pore structure when it is isostatically compressed at compaction pressures of at least 98 MN/m.sup.2 and by very high purity (at least 99.98% Al.sub.2 O.sub.3 plus possibly MgO excluding sulfate and particularly with a residual sulfur less than 0.5%) and by a specific area of at least 145 m.sup.2 /g.
The invention accordingly provides a means for obtaining homogeneous products comprised of very fine elementary particles and showing an improved compressability. The powders produced according to this invention are specifically useful in the manufacture of gaseous diffusion "barriers" for isotopic enrichment; for the preparation of ultrafiltration elements; and as loading minerals for particular paper products used to produce electrical condensers. The products of this invention are equally advantageously utilized for polishing metals and carbonaceous materials, and as catalysts or as catalyst supports.
The prior art technique of treating alum in circulating dishes in passage furnaces, for example, a tunnel furnace, does not generally permit obtaining products having surface areas greater than 125 m.sup.2 /g for residual sulfur contents below 0.5%. It is nevertheless well known that these products are not very homogeneous and that the specific area attributed to them must be considered as intermediate. This is easily understood by the inequalities of treatment undergone by the product according to whether it is situated at the top or the bottom of the crucible.
These differences are especially occasioned by the time lags in the heating schedule and the composition of the gas surrounding the product during its formation. These heterogeneities affect the porosity of the alumina. In effect, when for example the powder has been locally overheated, it may have formed some corundum. Now this crystalline transformation is always accompanied by a considerable increase in the grain size, and this gives rise to a macroporosity in the spectrum of the pores. By contrast, a region insufficiently calcined will still contain a high proportion of undecomposed sulfate and will show a very high specific area. Upon compression this heterogeneity causes an undesirable widening of the distribution of pore volumes.
Moreover, more productive, short residence time apparatus must operate at high temperature rapidly to cause decomposition of the sulfates. But the resulting overheating of the product does not permit in this case the attainment of high specific surface areas, and in addition, the resulting product is very heterogeneous.